It's staff wont be keeping patients/customers hanging around hours between adrenaline shots. That's the point. If a reaction occurs in a health provider setting it's highly unlikely they'd be sent packing the moment the reaction had subsided.
It beggars belief I'm having to state that the environment of a supermarket at the height of a pandemic is not a safe environment for a vaccine roll out, and that a pharmacy within it wont be as well stocked and staffed as an orthodox medical facility.
How about people being allowed to make up their own mind, when and where they get it. Rather than you trying to make it up for them, by using your usual feeble attempts at psychological coercion